In this work, a Finite Element calculations based on diffusion approximation are compared with Monte Carlo transport data code in time-resolved reflectance simulations of light propagation in a three-layered head model, which can be seen as a very simplistic approximation of the adult head. We also address the effects caused by the cerebrospinal fluid (CSF), filling the space between the skull and the brain, on the accuracy of the diffusion approximation for different values of CSF reduced scattering coefficients µs′ varying between 0.1 and 1 mm-1. Significant differences between transport and diffusion calculations show that diffusion approximation fails to describe accurately light propagation in voidlike region such as the cerebrospinal fluid (CSF), in which absorption and scattering are very small compared to the surrounding media, whereas the Monte Carlo predictions are not greatly affected. However, It is shown that the diffusion equation should provide reasonable solutions with a CSF reduced scattering coefficient µs′ = 0.3 mm-1. The results indicate that a multi-layered model including CSF is more appropriate for the determination of the optical properties of the human head and to obtain accurate solutions of the forward problem with diffusion approximation.
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